50L
I think getting accurate, sharp photos wide open has much more to do with the skill of the photographer than the limitations of his or her equipment.
True to a point. There is a technique to shooting at large apertures (part of that is knowing when to shoot wide open), but even with good techique, unless you're manually focusing, you're ultimately at the mercy of the AF drive to nail the focus with such a shallow depth of field (where the margin of error is extremely small)....the AF drive determines how consistently you're able to nail the focus, even with good technique. No amount of technique can correct the fact that the AF occasionally drive misses. I've owned the 50 1.8, 1.4, and 1.2. The 1.2 fairs better than the 1.4 and 1.8 in terms of auto focus accuracy, but even it is far from perfect.
The 85L is an example of a lens that has an extremely reliable, and extremely precise AF drive. It's AF drive is horrendously slow, but it very consistently nails the focus wide open, and very easily pops out sharp photos at an extremely shallow depth of field. My hit rate with the 50 mm's is much lower, though again the 50L fairs much better than the other two.
